We can create year_vec and day_vec to represent the position in the year and the position in the day.
Then, we can create datetime from them.

```python
import timevec.builtin_math as tv
import datetime
dt = datetime.datetime.now()
yv = tv.year_vec(dt)
dv = tv.day_vec(dt)
dt2 = tv.datetime_from_vec(dt.year, yv, dv)
assert dt == dt2  # This is true (but there may be a small floating point error)
```